Values.valueToJsonObject() shall return JSONNullValue instead of null

[metas-ts]

Is this a bug or feature request?

FR

What is the current behavior?

currently, this function might return null. For me that's a problem, because

• that way IVIewRow.getFieldNameAndJsonValues() does not return entries for null row values.
• therefore, those null row values are not part of the fieldsByNamemap that is send to the frontend.
• therefore, the respective row's viewEditorRenderMode isn't sent either
• therefore, the frontend falls back to a parent value, which is - in my case - wrong

Note that my particular problem could be handles in other ways, but i think if we have that method and we have JSONNullValue, then let's use it to avoid returning null

[metas-ts]

cherry-picked to 2018-06

[metas-lc]

found related issue when you pick more than you ordered https://github.com/metasfresh/metasfresh-webui-api/issues/847

[metas-rc]

Nothing to IT. Moving this to Done.